There is a significant interest in reducing contact resistivity (Rc) to improve the performance of semiconductor devices. For complementary metal-oxide-semiconductor (CMOS) transistors, Rc can contribute over 65% of the total on-state source to drain series resistance. One way to reduce Rc is to increase the junction doping level of the surface to be contacted. However, the junction doping is already near dopant solubility limits, so that further doping will not decrease Rc. Accordingly, the largest reduction in contact resistivity for sub-32 nm technology nodes available is likely to come from a reduction in Schottky barrier heights (SBH) at the Schottky barriers formed at the metal-semiconductor (MS) interfaces, where the SBH is the potential energy barrier for carriers at the MS interfaces formed at the source and drain contacts of the transistors.
At a metal/n-type semiconductor interface, the SBH is the difference between the conduction band minimum and the metal Fermi level. For a metal/p-type semiconductor interface, the SBH is the difference between the valence band maximum of the semiconductor and the metal Fermi level. The most common symbol used for the SBH is Φb, with Φbp (for metal contact to a p-type semiconductor) and Φbn (for metal contact to an n-type semiconductor). Silicides are a class of compounds that includes silicon with usually≧1 more electropositive metal element that provide the metal to form a MS interface with semiconductors such as silicon, such as at the source and drain of a MOS transistor.
One known solution to the silicide Rc issue is to use separate homogenous silicide materials (an “NMOS silicide” and a “PMOS silicide”) for contact to the p-type semiconductor regions (the PMOS silicide) and n-type semiconductor regions (the NMOS silicide). As an example, Yb can be included in Nisilicide to provide a first homogenous silicide material (NMOS silicide) which can lower the electron SBH of the silicide for contacts to n-type Si, but not for contacts to p-type Si as it raises the hole SBH to p-type Si compared to Nisilicide. Accordingly, Pt can be included in Nisilicide to provide a second homogenous silicide material (a PMOS silicide) which lowers the hole SBH of the silicide on p-type Si (but raises the electron SBH on n-type Si) for contacts to p-type Si. However, this process adds a masking level to create the homogenous NMOS silicide and homogenous PMOS silicides independently, which also adds to process complexity and cost to manufacturing.